To meet the demand for ever-increasing wireless data traffic since commercialization of the 4th generation (4G) communication system, there have been efforts to develop an advanced 5th generation (5G) or pre-5G communication system. The 5G or pre-5G communication system is called a beyond 4G network communication system or post LTE system.
Implementation of the 5G communication system using ultra-frequency (mmWave) bands, e.g., 60 GHz bands is considered to achieve higher data rates. To reduce propagation loss of radio waves and increase a transmission distance in the ultra-frequency bands, beamforming, massive multiple-input multiple-output (MIMO), full dimensional MIMO (FD-MIMO), array antenna, analog beamforming, large-scale antenna techniques are under discussion.
To improve system networks, technologies for advanced small cells, cloud radio access networks (RANs), ultra-dense networks, device to device (D2D) communication, wireless backhaul, moving networks, cooperative communication, coordinated multi-points (CoMP), reception-end interference cancellation and the like are also being developed in the 5G communication system.
In addition, in the 5G system, an advanced coding modulation (ACM), e.g., hybrid frequency-shift keying (FSK) and quadrature amplitude modulation (QAM) modulation (FQAM), sliding window superposition coding (SWSC), and an advanced access technology, e.g., filter bank multi carrier (FBMC), non-orthogonal multiple access (NOMA), sparse code multiple access (SCMA) are being developed.
Recently, a series of studies for operating a Long Term Evolution (LTE) system in an unlicensed frequency band have been actively underway. The LTE system operating in an unlicensed frequency band is referred to as an LTE-U (LTE-Unlicensed or Unlicensed LTE) system.
An unlicensed frequency band is not a frequency band allocated to only one specific system. Therefore, when any communication system is licensed for use by a regulatory authority such as the Federal Communications Commission (FCC), and complies with the rules set by the regulatory authority, communication can be performed by transmitting or receiving a wireless signal through an unlicensed frequency band. Wi-Fi and Bluetooth, which are currently used by many people, correspond to a typical technology that uses an unlicensed band.
Recently, as mobile data traffic through LTE and LTE-Advanced (LTE-A) has significantly increased, telecommunication service providers have shown great interest in securing a frequency band that can accommodate the mobile data traffic. The most basic method therefor is to purchase a licensed frequency band allocated for an LTE system. However, since frequency resources are a public resource which is strictly managed and controlled by a nation, purchasing a licensed frequency band requires a lot of time, costs and complex procedures. Therefore, some communication service providers and communication chip manufacturers try to operate an LTE system in an unlicensed band near 5 GHz. In the 5 GHz band, an LTE system is known to be able to use a wide frequency band of about 500 MHz. Therefore, it is expected that the effective use of an unlicensed frequency band of 5 GHz will greatly increase the capacity of an LTE system.
However, an unlicensed frequency band near 5 GHz can be used for an LTE system as well as other communication systems (typically, a Wi-Fi system). Therefore, a method for efficiently occupying a channel by competing with communication apparatuses of other communication systems for channel occupation while following all the necessary regulations in a corresponding unlicensed frequency band, and a downlink scheduling method are required.
In a communication system using an unlicensed frequency band, data scheduling is possible after Listen-Before-Talk (LBT). The LBT refers to a scheme of communicating only when a usable channel is detected before data transmission or reception and a Clear Channel Assessment (CCA) condition (meaning that only energy less than or equal to −82 dBm is generally detected, differing with countries) is satisfied. However, in an LTE system, a physical downlink control channel (PDCCH) is transmitted to the front of every sub-frame and data scheduling is possible through the PDCCH. Therefore, it can be seen that a scheduling procedure of a general LTE system for data scheduling through a PDCCH, without checking whether a channel is available, is not suitable for an LTE-U system. Therefore, in order for LTE-U to operate in an unlicensed band, unlike a Wi-Fi system, a base station should listen to a corresponding unlicensed band and identify whether the CCA condition is satisfied. When the CCA condition is satisfied, the base station should continuously transmit a preservation signal to occupy a channel until PDCCH signals are transmitted to user equipments. Accordingly, the base station wastes an extreme amount of power due to the transmission of the preservation signal for a relatively long time, and this may interfere with channel occupation of other communication apparatuses communicating in an unlicensed band. Furthermore, the overall channel utilization efficiency in an unlicensed band also decreases.